1. Field of the Invention
This invention relates to a method for extracting oil from oil-bearing plant parts such as seeds, fruits, nuts, leaves, germ, bran, bark, and roots and, more particularly, to a method for extracting these organic oils at sufficiently low temperatures (less than 160.degree. F.) so as to not denature the proteins, vitamins and minerals, resulting in two useful and marketable substances, the oil and defatted meal.
2. Background
When harvested, rough rice, also called paddy rice or "paddy", consists of a starchy rice kernel surrounded by a tightly adhering brown coating of bran and enclosed within a loose outer hull or husk. In current commercial practice, the paddy is first dried and then milled. In a mechanical milling process, the hull and bran are removed to obtain white rice. The by-products of milling are rice hulls and rice bran. Rice hulls are mainly cellulose, lignin and minerals with no significant food or feed value. For the most part, hulls, which average by weight about 20% of the paddy, are discarded as waste material or used as a low-value soil conditioner, fuel or crude abrasive.
Rice bran, on the other hand, is rich in protein (13 to 16%) and food energy and contains high levels of natural vitamins and essential trace minerals. These qualities have led to a high demand for rice bran as an animal feed ingredient, and it is used extensively for this purpose throughout the world. Bran represents by weight 4 to 9 percent of the paddy, varying with location of cultivation and degree of milling.
Under normal conditions, when brown rice is milled to white rice, the oil in the bran (16 to 22%) and a potent lipase, also in the bran, come into contact with each other. This results in a rapid degradation of the oil to glycerol and free fatty acids. The bran thus produced is unpalatable and is not used as a foodstuff. Rather it is used as animal feed.
Hexane is conventionally used as a solvent for the extraction of rice bran oil from rice bran. Use of hexane poses potential air pollution problems. More importantly, hexane solvent extraction requires high operating temperatures (greater than or equal to approximately 160.degree. F.) which denature or downgrade the functional and nutritional properties of rice bran and rice bran oil. Thus, some means of extracting rice bran oil from rice bran that does not denature or downgrade the functional and nutritional properties of rice bran and rice bran oil is desirable. More specifically, solvent extraction that does not require high operating temperatures (above approximately 160.degree. F.), which result in denaturing of rice bran and rice bran oil, is desired.
Other solvents, such as propane, ethane, carbon dioxide, dinitrogenoxide, butane and isobutane, have been used for extraction of oil from oil bearing plant material, but either not at sufficiently low operating temperatures to avoid denaturing of proteins, vitamins and minerals or else using separate vessels to perform the dissolving and extraction steps, or both.
Conventionally, various methods exist for extracting oils from organic material, including the use of liquid hydrocarbons, for example, U.S. Pat. Nos. 4,331,695; 2,560,935; 183,097; 183,098; 2,485,916; 2,571,948; 2,727,914; 3,261,690; 3,271,160; 3,492,326; 3,542,559; 3,852,504; 4,457,869; 4,486,353; 2,548,434; and 1,802,533. U.S. Pat. No. 4,331,695 to Zosel discloses propane and other hydrocarbons as solvents; however, Zosel utilizes extremely high pressures (approximately 42 atmospheres), the extraction is done at high temperatures (80.degree. C. to 176.degree. C.) which would denature proteins, and appears to require constant pressure with a potential temperature swing from 0.degree. C. to 100.degree. C. The '935 patent discloses propane as a solvent at high temperature (substantially above 140.degree. F.) in a two-stage extraction process. The '097 and '098 patents introduce water into the process.
The '916 patent discloses a pre-extraction process involving alcohols and soap at a temperature between 51.degree. and 212.degree. F., followed by heating almost to the boiling point of the solvent.
The '948 patent discloses steam distillation. The '914 patent discloses cooking rice bran prior to solvent extraction at a temperature above 170.degree. F. The '690 patent uses superheated steam to extract the oil from the solvent. The '160 patent discloses processing safflower seed residue after the oil is extracted.
The '326 patent discloses liquid hydrocarbon solvent extraction for extracting not more than 10% by weight of rice oil from rice bran at an entering temperature, using hexane, of between 120.degree. F. and 160.degree. F., subjecting brown rice to temperature between 125.degree. F. to 130.degree. F.
The '559 patent discloses hexane or liquid hydrocarbons at between 115.degree. and 120.degree. F. The '281 patent discloses extraction at both high temperature and high pressure. The '869 patent discloses high temperature extraction using isopropyl and isopropanol based solvents. The '353 patent uses ethanol at 158.degree. F. and above.
The '434 patent discloses gaseous hydrocarbons at high pressure and high temperature (150.degree. F. to 200.degree. F.). The '533 patent discloses butane, propane and other hydrocarbons 80.degree. F. and 30 psig or 60.degree. F. and 15 psig, but the meal is placed in a separate still where it is heated to remove solvent.